Achiral Surface Promotes Chiral Amplification

The curious observation that certain biochemical building blocks found in living organisms come almost exclusively in just one enantiomeric form—L for amino acids and D for sugars—has led scientists to propose mechanisms explaining its origins. Researchers at Carnegie Mellon University have now added another piece to this homochirality-of-life puzzle. Yongju Yun and Andrew J. Gellman determined that adsorption of gas-phase chiral molecules on an achiral surface can amplify the enantiomeric excess of the mixture (Nat. Chem. 2015, DOI: 10.1038/nchem.2250). For years, scientists have studied the role that adsorption processes may play in enantioselectivity. But until now the focus has been on chiral surfaces, including naturally occurring chiral minerals such as quartz. Surprisingly, Yun and Gellman found that a chiral surface is not required for adsorption-based enantiomeric enrichment. The team exposed an achiral copper surface to a ¹³C-labeled gas-phase mixture of D- and L-aspartic acid that was slightly enriched in the d isomer. On the basis of mass spectrometry analysis of adsorbed enantiomers and numerous control experiments, the team showed that adsorption on the achiral surface increased the d isomer’s enantiomeric excess from roughly 30% to nearly 90%.